Adsorbent carbon, particularly activated carbon, is used extensively for a variety of purposes, many of which depend to a considerable extent on the large surface area and surface activity of the carbon particles. This adsorbent property of carbon is the basis for its use in many filter applications, including those for filtering and removal of certain fluids, particularly gases. The ability of adsorbent particles, particularly activated carbon particles to remove impurities from fluids, generally, and gases, such as air, in particular, has led to their extensive use in filters of varying types, including air conditioning filters, heating plant filters, gas masks and the like. Activated carbon, the material preferred for use in gas masks and the like, has an extremely high porosity and is characterized primarily by a very large specific surface area.
Although the manner in which a filter made of adsorbent, or activated, carbon retains substances which come in contact with the carbon particles is not fully understood, it is believed that retention occurs largely by means of physical sorption, chemical sorption and catalytic reactions. Specificity and efficiency of retention of certain substances has been shown to be increased by treatment of carbon particles with different metal compounds, such as in a process known as Whetlerizing, to produce activated carbon impregnated with such metal compounds, the product known as Whetlerite or ASC carbon. This type of metal compound-impregnated activated carbon is particularly useful in gas masks and the like where removal of toxic gases and biological materials is critical. In particular, Whetlerite is quite effective in removing hydrogen cyanide and cyanogen chloride. These gases are not removed by simple adsorption but react with the metal compounds present in the Whetlerite to produce non-toxic products.
Although highly effective in applications such as those indicated above, Whetlerite has been found to be prone to a loss of chemical reactivity toward such materials as hydrogen cyanide and cyanogen chloride. The loss of reactivity is most pronounced under conditions of elevated temperature, high humidity, and in the presence of oxygen. Thus, a relative humidity of about 65 to 100 percent and temperatures of about 80 degrees F. or higher for protracted periods of time result in loss of chemical reactivity toward hydrogen cyanide and cyanogen chloride gases. Contact with oxygen, particularly at concentrations of about 25 percent or greater at elevated temperatures, particularly at about 200 degrees F. or greater has been found to be especially detrimental to ASC carbon.
Some of the metal compounds employed in Whetlerite activated carbon, although having relatively low vapor pressures, are potentially toxic if inhaled or ingested. Some of these compounds, such as chromium salts, have been shown to be carcinogenic. In many instances, adsorbent carbon powder which has been treated with such metal compounds for use in gas masks and the like, may contain carbon fines either as a result of natural particle size distribution or from fines generation from handling during processing. When gas masks employing such treated adsorbent carbon powder are manufactured, there exists a potential danger of the metal salt impregnated carbon fines passing through the filter and being absorbed or inhaled by a person wearing such a gas mask. Thus, the wearer may avoid one health hazard only to face another potential health hazard. With such fines present in the filter of a gas mask, there is also the problem of inhalation of toxic material or toxins adsorbed by the fines in the filtration process.